Reactive filtration

ABSTRACT

In one embodiment, a reactive filtration method includes continuously regenerating a reactive filter media while simultaneously filtering contaminants from fluid flowing through the filter media. In one embodiment, regenerating the reactive filter media comprises mixing metal granules with the filter media and agitating the mixture. In another embodiment, regenerating the reactive filter media comprises introducing a metal in the fluid flowing through the filter media and agitating the filter media. In one embodiment, a method for removing phosphorus, arsenic or a heavy metal from water includes introducing a metal salt reagent into the water at a molar ratio of 5:1 to 200:1 to the phosphorous or the arsenic in the water and passing the water through a bed of moving sand.

CROSS REFERENCE TO RELATED APPLICATION

The present patent application is a divisional of, and claims priorityfrom, U.S. patent application Ser. No. 10/727,963, filed Dec. 3, 2003now U.S. Pat. No. 7,399,416 and entitled “Reactive Filtration” whichclaims the benefit of U.S. Provisional Patent Application 60/430,756,filed Dec. 4, 2002. The disclosures of the above mentioned patentapplications are incorporated herein by reference in their entireties.

STATEMENT OF RIGHTS TO INVENTIONS MADE UNDER FEDERALLY FUNDED RESEARCHAND DEVELOPMENT

Part of the work performed during the development of embodiments of theinvention was funded by the United states Environmental ProtectionAgency under contract no. EPA-EPSCoR GR827683-01-0. The United Statesgovernment may have certain rights in the invention.

BACKGROUND

Phosphorus exists in waters as dissolved ortho-phosphate, polyphosphate,and complex organo-phosphorus compounds. In typicalphosphorus-containing waste waters, such as the secondary or tertiaryeffluents of municipal waste water treatment plants, there is adissolved fraction of phosphorus compounds, primarily in the form ofortho-phosphate and poly-phosphates, and a suspended fraction ofmicro-particulate phosphorus-containing solids. Trace levels of arsenicare sometimes found in some sources of drinking water and in higherconcentrations in some waste waters. Arsenic can occur in natural watersas reduced arsenite, As(III), or oxidized arsenate, As(V).

Several methods are currently utilized for the removal of phosphoruscompounds, arsenic, and other contaminants from waste water.Micro-particulate and other solid contaminants are typically removed byfiltration using a solid media such as sand, and sedimentation, wheresolid contaminants with higher densities than water are allowed tosettle. Dissolved contaminants are typically removed by flocculation andsorption. In flocculation, metal salt solutions are mixed with wastewater to precipitate the contaminant out of solution, where it can thenbe removed through filtration or sedimentation. In sorption,contaminated waste water is passed through a stationary filtrationmedia, typically iron oxide coated sand, having a partially chargedcationic boundary layer that is reactive with a target contaminantdissolved in the waste water.

In conventional fixed-bed filtration systems, filtration media canquickly lose its filtration efficiency as the interstitial spacesbetween the particles of the filtration media become saturated withmicro-particulate and solid contaminants. Thus, the filtration mediamust be flushed or replaced, which tends to be costly and timeintensive. Additionally, the filtration process must be stopped whilethe filtration media is being flushed or replaced.

Moving-bed filtration devices seek to mitigate these limitations byutilizing processes that remove micro-particulate and solid contaminantsfrom the filtration media while simultaneously filtering water. Thesemoving-bed filtration devices still have a disadvantage in that they donot remove dissolved contaminants from waste water.

U.S. Pat. No. 5,369,072 to Benjamin et al. describes methods ofpreparing iron-oxide coated sand to be used as a filtration media ineither fixed-bed or moving-bed systems to remove both solid anddissolved contaminants from waste water. When such a filtration media isused in a fixed-bed system, there remains the disadvantage of having toflush or replace the filter media on a regular basis to remove the solidcontaminant waste. Although the use of this media in a moving-bed systemmay overcome these disadvantages, it does not overcome a disadvantagecommon to the use of the described iron oxide coated sand in eithersystem. That is, the reactive surface of the iron oxide coated sandbecomes saturated with adsorbed contaminants, and therefore needs to beeither replaced or regenerated. Benjamin states that the adsorbedcontaminant can be desorbed by treating the saturated filtration mediawith a solution with a pH range that is known to desorb the specificcontaminant ion from the specific adsorbing surface. Therefore, ineither a fixed-bed or moving-bed system, filtration of water must stopto allow the filtration media to be rinsed with pH solution.

Environmental concerns and increasingly stricter government regulationshave many industries searching for cost-effective and efficientwater-treatment solutions. Additionally, there is a recognized need tobe able to filter out water contaminants that exist in what may beconsidered trace amounts. For example, the removal capability ofconventional coagulation-precipitation methods drop off significantly atcontaminant levels lower than 500 parts per billion (ppb). However,science has recognized that ambient phosphorus levels in water greaterthan 10-20 ppb can lead to eutrophication. The U.S. EnvironmentalProtection Agency is pushing for lower limits of phosphorus ineffluents. Currently, the EPA estuarine water criteria for totalphosphorus is 0.10 mg/L. High volume dischargers are experiencing areduction in the levels allowed in their regulatory permits.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a moving bed particle filtration system that may beused to implement various embodiments of the invention.

FIG. 2 illustrates a moving bed filtration system constructed accordingto one embodiment of the invention in which the waste water ispre-treated with a reagent.

DETAILED DESCRIPTION

Embodiments of the invention were developed in an effort to moreefficiently remove contaminants from waste water by increasing flowsthrough a filtration system, even when contaminants are present inrelatively small amounts. “Waste water” as used in this Description andin the Claims means any water to be treated—it is not necessarily highlycontaminated water and may contain only trace amounts of phosphorus,arsenic, or other contaminants (organic or inorganic and in single ormixed solution).

FIG. 1 illustrates a moving-bed particle radial filtration system 10that may be used to implement various embodiments of the invention.Referring to FIG. 1, waste water flows into a vertically orientedcylindrical treatment vessel 12 through an inlet pipe 14. Vessel 12includes a filter chamber 16, a stem 18 and an expansion gravitysettling chamber 20. Filter chamber 16 contains a bed of sand 22, ironoxide coated sand, sand and iron granules or another suitable filtermedia. Inlet pipe 14 extends down into filter chamber 16. Waste water isdischarged into sand 22 along the perforated lower part 24 of inlet pipe14. Treated water flows out of filter chamber 16 through a perforatedouter perimeter 26 into a sleeve 28 and is removed from vessel 12through an outlet pipe 30. The perforations in the lower part 24 ofinlet pipe 14 and the outer perimeter 26 of filter chamber 16 arescreened as necessary to prevent sand from passing through theperforations.

The comparatively narrow stem 18 of vessel 12 connects filter chamber 16with expansion chamber 20. A sludge removal port 32 is positioned nearthe bottom of expansion chamber 20. A recirculation pipe 34 extends fromthe bottom of filter chamber 16 to the top of expansion chamber 20. Anair compressor 36 pumps air into recirculation pipe 34 at the bottom offilter chamber 16 causing a counterclockwise motion of air, water, sandand filtered particulates through vessel 12. A back flow preventer 38,such as a flapper valve, prevents materials in recirculation pipe 34from flowing back into compressor 36. A flow control valve 39, samplingtube 40, sampling valve 42 and clean-out 43 on recirculation pipe 34,and a sight glass 44 in stem 18, may be provided if necessary ordesirable.

In operation, waste water pumped into filter chamber 16 through inletpipe 14 passes radially through sand 22 into sleeve 28 and flows outoutlet pipe 30 as treated water. Sand 22 moves continuously down throughvessel 12 under the influence of gravity. An aerated mixture of usedsand and water flows from the bottom of filter chamber 16 back up toexpansion chamber 20 through recirculation pipe 34 along withcontaminants removed from the waste water. Air is vented to theatmosphere at the top of expansion chamber 20 to prevent pressurizationof the system. The pressure head of water in sand 22 is kept such thatsome of the treated water flows from filter chamber 16 up through stem18 into expansion chamber 20 to rinse contaminants from the used sandparticles returning to expansion chamber 20. This rinse water, nowcarrying a high concentration of contaminants less dense than sand, isremoved from chamber 22 and flows out through sludge removal port 32. Ina preferred operation, the top of the sand bed for filtration is threefourths the height of filter chamber 16. Expansion chamber 20 and narrowstem 18 contain a dilute sand and water mixture that contains filteredparticles that have been moved first to the bottom of sand 22 andcirculated via pipe 34 into the water residing in expansion chamber 20.Water flow at inlet pipe 14, outlets 30 and 32 and recirculation pipe 34can be balanced so that a preferred rate of 5-10% of the inlet watercarrying contaminants is discharged through sludge removal port 32.

The system of FIG. 1 may be used to implement a process for continuouslyregenerating an iron oxide coated sand bed while simultaneouslyfiltering contaminants from the incoming flow of waste water. Theprocess creates and utilizes a reactive filter media that removescontaminants by filtering and by adsorption. A reactive filter media isany filter media with the additional capability of removing contaminantsfrom waste water through chemical processes such as adsorption. The ironoxide coated sand bed, a reactive filter media, screens contaminantsfrom the water and the reactive surfaces of the granules of sand adsorbcontaminants from the water. In one embodiment, iron metal granules inproportions of 10-30% by volume in sand bed 22 provide a solid phasereactive surface of corroding iron metal as well as a source ofdissolved iron such as salts of Fe (II) and Fe(III) that react with thesand in the filter bed to create reactive iron oxide coated sand. Thestrongly reducing nature of water solutions with iron metal and sandmixtures can be useful for chemical reactions, such as the reductivedegradation of organic solvents dissolved in contaminated water.Reduction potentials lower than −200 mV versus the standard hydrogenelectrode can be observed with 30% iron:sand mixtures.

In an alternative embodiment, a reagent capable of creating a reactivesurface on the filter media is added to the incoming flow of waste waterat molar ratios such as 5:1 to 200:1 with the target contaminant. Whileit is expected that soluble forms of manganese, aluminum or other metalssuch as zinc and copper will provide suitable reagents, iron willtypically be used as the reagent due to its proven reactivity with avariety of contaminants and its current widespread use in watertreatment. Ferric chloride, for example, is a preferred reagent whenphosphorus or arsenic is the target contaminant. In any particular watertargeted for treatment, their may be alternate and competitive reactivepathways for the added active reagents. These pathways will be theresult of the specific water chemistry in the waste water. For example,waste water with high levels of dissolved carbonate or phosphate canreact with added iron salts in competition to the target contaminantsuch as arsenic. Molar ratios of Fe(III) to water arsenic in fieldstudies have been in excess of 100:1. In these studies, inletconcentrations of arsenic in source water for drinking were reduced fromapproximately 40 parts per billion to less than 5 parts per billiontreating at a rate of 10 gallon per minute in a pilot scale operation.However, other water types may have less alternate, competitive reactivepathways. It is preferred to field test to determine the optimal molarratio for any particular treatment environment to ensure sufficientexcess reagent is delivered to the reactive sand surface to form ironoxide coated sand. Additional considerations in reagent balancing directefforts to minimizing reagent addition to ensure that the processeffluents are not overly high in dissolved iron or other reagent,thereby creating an additional treatment or discharge concern. Excessreagent consumption will also undesirably increase the cost of operationof the process.

In the removal of dissolved and suspended phosphorus, field studies havedemonstrated that successful high flow, low concentration removal occursin this process in iron to phosphorous molar rations of 5:1 to 40:1. Itis preferred that the actual reagent dose is optimized to ensure nearcomplete solution reaction and saturation of all of the competingreactive pathways and allowing for residual iron in the solution toreact with the sand bed. In some phosphorus contaminated test wastewaters, optimizing the correct balance of conditions yields a preferredratio of iron to phosphorus at 8:1. The metal salt reagent, ferricchloride in this example, reacts with the surface of the sand to formiron oxide coated sand (IOCS). IOCS provides a stationary phase removalpathway for water borne contaminants such as phosphorus and arsenic.Contaminants in the waste water are exposed as a “mobile” phase over the“stationary” (slowly moving) IOCS bed for high efficiency sorptive andion exchange removal. The physical action of the moving sand abrades thesurface of the sand granules, regenerating active sites for additionaliron salt and water contaminant reactions. Hence, regenerated reactivesites for contaminant binding are continually presented to the flowingwater. Abraded sand-iron-contaminant solids are removed by the screenfiltering action of the sand bed. The treated water exits the sandfilter bed with contaminants substantially removed, ready for dischargeor post-treatment processing.

Sorption is the removal of undersaturated solutes from solution ontominerals. Sorbate is the species removed from solution and the sorbentis the solid onto which solution species are sorbed. There are threetypes of sorption: adsorption wherein solutes are held at the mineralsurface as a hydrated species; absorption wherein solute is incorporatedinto the mineral structure at the surface; and ion exchange wherein anion becomes sorbed to a surface by changing places with a similarlycharged ion previously residing on the sorbent. Mineral surfaces, suchas the silicates in sand, have fixed or acquired surface charges thatcan be modified by water chemistry such as pH and dissolved solutes suchas iron salts that can complex with the surface charges of sand. As aresult of fixed surface charges, a property of the mineral, and pH, aproperty of the water, mineral surfaces develop a point of zero netproton charge (PZNPC). The PZNPC is the pH at which net surface chargeis zero. At lower pH than PZNPC, the net surface charge is positive andat higher pH, the net surface charge is negative. These surface chargesallow attraction of oppositely charged anions or cations, respectively,from solution. Larger amounts of dissolved constituents, such aspositively charged Fe(III) can be attracted to a negatively chargedsurface such as the silicates in sand to such a degree that the surfacebecomes overall positively charged and therefore attractive to anionssuch as phosphate and arsenate. Silica, SiO₂ has a low PZNPC of 2,whereas iron oxyhydroxide, α-FeOOH has a PZNPC of 7.8, and ironhydroxide, Fe(OH)₃ has a PZNPC of 8.5. Increasing quantities of ironoxide forming on a sand surface will increase the PZNCP of the sandgrains such that net surface charge is positive and thereby attractiveto anions such as phosphate and arsenate at higher pH levels of about6-8. Most environmental waters, including drinking water and wastewatersexist at these circum-neutral pH levels. Hence, the selective additionof iron oxides to the sand creates a useful sorbent.

In a moving sand bed system such as the one shown in FIG. 1,concentrated contaminants, now in the form of filterable solid waste,are removed from the system through sludge removal port 32 viacontinuous rinsing in expansion chamber 18. This continuous rinsing andwaste removal process is particularly important in the case of ahazardous material such as arsenic in drinking water. Rinse/waste wateroutflow, typically 5-10% of the incoming water, can be recycled and putback into the process following separation of the suspended solids bysettling or clarification. In a fixed-bed system, in which theparticulate filtrate remains on the sand and in the sand, the sand bedis periodically flushed or changed out to remove the concentratedcontaminant waste.

FIG. 2 illustrates a novel moving bed filtration system 50 constructedaccording to one embodiment of the invention in which the waste water ispre-treated with a metal salt reagent. Referring to FIG. 2, filtrationsystem 50 includes a pre-reactor system 52 and a reactive filter system54. Waste water is pumped into the serpentine piping 56 of pre-reactor52 through an inlet pipe 58 and flow control valve 60. A metal salt orother suitable reagent is introduced into serpentine piping 56 through areagent inlet port 62 immediately downstream from inlet pipe 58.Preferably, serpentine piping 56 is substantially larger than inlet pipe58 to slow the flow through piping 56 compared to inlet pipe 58. Aslower flow increases the time available for the reagent to mix with thewaste water and react with contaminants in the waste water. The wastewater flow will be more turbulent near the transition from the smallerinlet pipe 58 to the larger serpentine piping 56. Introducing thereagent into this turbulent flow also helps mixing.

The waste water/reagent mix flows through straight-aways 64 and gentlebends 66 of serpentine piping 56. The waste water/reagent mix exitsserpentine piping 56 into an outlet pipe 68 that takes the mix intoreactive filter system 54. Prescribed overdosing introduces the reagentin sufficient quantities and concentrations to (1) allow for theco-precipitation and flocculation reactions between the reagent and thedissolved contaminants in pre-reactor system 52 to go to near completionto dilute levels where equilibrium and diffusion limited processes limitfurther reaction, (2) saturate competing reactive pathways with naturalwaters with reagent, and (3) leave enough excess reagent in the mix toactivate the filter media in reactive filter system 54. The amount ofexcess reagent is determined by the reactive capacity of the influentsolution and the desire to deliver excess reagent to the sand filtrationbed for the continuous formation of iron oxide coated sand.

The comparatively slow flow through serpentine piping 56 allows forbetter coagulation of precipitates. The straight-aways 64 allow for lessturbulent flow to enhance coagulation. Periodic gentle bends 66introduce and maintain additional turbulent flow and introduce flowvortices to periodically mix the flowing solution. Preferably, theserpentine mixing array allows for a decrease in flow velocity for 2-8minutes to allow for sufficient pre-reaction time. Design of the arrayneeds to consider maintaining sufficient flow to prevent deposition ofprecipitation solids in the pre-reactor assembly. The actual length anddiameter of serpentine piping 56 for most applications will result foran optimization of the required reaction time (usually 1-5 minutes), thedesired flow rate, the space available at the site of deployment, andthe presence of competing reactions in the treatment water.

The pre-treated waste water flows into the vertically orientedcylindrical treatment vessel 70 of reactive filtration system 54 throughan inlet pipe 72. Inlet pipe 72 is positioned at the center of vessel70. Vessel 70 includes a filter chamber 74 that contains a bed of sand76 or another suitable filter media. Inlet pipe 72 extends down intofilter chamber 74 to discharge the waste water into the lower portion ofsand bed 76 through a perforated manifold 78. Waste water pumped intofilter chamber 74 passes up through sand 76, over a baffle 80 near thetop of filter chamber 74 as fully treated water, into a basin 82 and isremoved from vessel 70 through an outlet pipe 84.

A recirculation tube 86 extends from the bottom to the top of filterchamber 74 at the center of vessel 70. Inlet pipe 72 extends down thecenter of recirculation tube 86. Inlet flow discharge manifold 78extends out through openings in recirculation tube 86. An air compressor88 pumps air into used sand and water at the bottom of vessel 70 throughan air inlet pipe 89. The aerated mixture of used sand and water risesthrough recirculation tube 86 along with contaminants removed from thewaste water up to a sand and particulate/water separator 90. Separator90 represents generally any suitable separation device that may use, forexample, physical separation, gravity separation, particle sizeseparation, magnetic separation, membrane separation, or cyclonicseparation. The sand removed from the mix by separator 90 is recycledback to filter chamber 74. The now highly contaminated waste water isremoved through a sludge removal port 94. Sand 76 moves continuouslydown through vessel 70 under the influence of gravity.

Phosphorus exists in waters and waste waters as dissolvedortho-phosphate, polyphosphate and complex organo-phosphorus compounds.In typical phosphorus containing waste waters, such as the secondary ortertiary effluents of municipal waste water treatment plants, there is adissolved fraction, primarily as ortho-phosphate (PO₄ ³⁻) andpoly-phosphates and as a micro-particulate or suspended fraction ofphosphorous containing solids. Trace levels of arsenic are sometimesfound in some sources of drinking water and in higher concentrations insome waste waters. Arsenic can occur in natural waters in the reducedarsenite, As(III) or oxidized arsenate, As(V) forms. Arsenate reactswith iron and aluminum salts to form insoluble compounds. Waters witharsenite contamination can be treated with an oxidizer such as chlorineto allow for further reaction with reactive metal salts. Ferric chlorideor sulfate is typically used as a metal salt reagent to removephosphorus and arsenic from water, although other salts and ferrouscompounds can be used.

In the system described above, excess ferric iron enters sand bed 76along with the particulate Fe—As or Fe—P solids and residual As or P insolution in the waste water. Ferric ions react with sand surfaces toform iron oxide coated sand (IOCS). IOCS sorbs residual solution As/Pout of solution. The physical action of the moving sand abrades thesurface of the sand granules, refreshing active sites for additionalIOCS formation and Fe—As or Fe—P reactions. Hence, fresh reactive sitesfor As/P binding are continually presented to the flowing water viamicroscopic erosion of the sand surface.

For phosphorus, ferric chloride is added at a preferred molar ratio of5:1 to 40:1 with the phosphorus in the waste water. The pre-reactorsystem allows for a pre-reaction to form metal phosphate salts such asFePO₄, Vivianite and humic-fulvic organic phosphorus solids that areamenable to filtration in the sand bed reactive filter system. Vivianiteis a very thermodynamically stable compound that is rapidly formed insolutions of iron cations and phosphate. Excess iron salt reagent ispassed unreacted into the sand bed where it binds to the surface of thesand to form iron coated sand, a phosphate and polyphosphate reactivesurface. Metal cations will selectively bind to the silicate and othernegatively charged groups on the solid sand surface. This binding willyield a partially charged cationic boundary layer on the iron coatedsand surface that will be reactive with soluble ortho-phosphate andpoly-phosphate. The mobile phase (treatment water) and stationary phase(iron coated sand) configuration of this process allows for nearquantitative removal of phosphorus because diffusion processes arenearly eliminated in the dilute solution reactive pathway of thisprocess. Testing has shown that this process can remove ortho-phosphateto less than detection limits (10 parts per billion) at efficienciesgreater than 99% and total phosphorus to less than 40 parts per billionat greater than 90% efficiency of removal from the originalconcentration.

The processes described above have been shown to produce iron arsenicsolids that are classified non-hazardous by the Toxicity CharacteristicLeaching Procedure (TCLP) directed by the Resource Conservation andRecovery Act (RCRA 42 U.S.C. s/s 6901 et seq.) and can be disposed in alandfill, and iron phosphate solids that may be used in agriculturalapplications as a low grade slow release fertilizer.

The reactive filter media are deployed in a moving bed to assist incontinuous renewal of the reactive iron oxide layer. Movement may beaccomplished, for example, by fluidizing or moving the bed using thefluid flow, by mechanical action such as augers or mixing bars, byacoustic action such as the application of ultrasonic waves or byphysical transport using compressed air.

Other embodiments are possible. For example, the filter media can be anynatural or synthetic, organic or inorganic substrate that can react withdissolved iron to form a reactive oxide surface. The particle size ofthe filter media will be a size suitable for the level of filtration andflow desired. It is expected that the following inorganic materials willprovide suitable filtration media: sand; silica beads or granules; highsilicate glass; glass beads; glass sand; zeolite; mineral sands such asolivine, hematite, goethite; diatomaceous earth; iron oxyhydroxidegranules; iron oxide granules; ceramic beads or granules; iron metalgranules or beads; iron metal coated beads or granules; and synthetic ornatural iron coated sand. It is expected that the following organicmaterials will provide suitable filtration media: polystyrene beads;polyethylene beads; modified cationic surface polymer beads; modifiedanionic surface polymer beads; mixed or pure polymer beads or granules;and polymer coated inorganic beads or granules. Some of these materialscan have naturally occurring reactive sites that can be maintained orsupplemented by the addition of active reagents such as ferric chloridesolution. Because of the well known filtration properties of sand, itsinexpensive use, its routine application in water treatment, its naturalreactive silicate surface for inner sphere and outer sphere metal oxidebinding to form iron oxide coated sand, and its abrasion properties, itis the preferred embodiment of an active filtration media in a movingbed process.

Suitable filtration media include corroding iron metal granules or ionexchange resins with the ability to bind iron compounds. Corroding ironmetal granules allow for reductive processes that can be used to removetrace amounts of chlorinated solvents in water. Testing has shown that a30% by volume iron-sand bed deployed in the system of FIG. 1 has asolution oxidation-reduction potential of −200 mV versus the standardhydrogen electrode. Typical deployments of static beds of iron granulesor iron granules and sand suffer from loss of porosity or passivation ofthe reactive iron metal surface. The motion of a moving bed deploymentallows for a continual refreshing of the iron metal surface and itsassociated chemically reactive sites as well as maintenance offiltration ability. A 98% efficiency has been demonstrated for removingphosphorus from contaminated discharge water originally containing 2 to3 parts per million phosphorus.

In some circumstances, removing arsenic for example, it may be desirableto pre-oxidize the waste water to convert arsenite to the iron reactivearsenate. Arsenite in natural solutions at circumneutral pH is non-ionicand therefore typically non-reactive in most treatment deployments.Pre-oxidation can be accomplished using conventional water oxidationoperations such as chlorination, sonication or ozonation. Thepre-oxidation operation can be part of a full water treatment processtrain and may be preceded or followed by other conventional watertreatment processes such as filtration, aeration, chemical treatment,flocculation, clarification and others that may be required in thenormal processing and disinfection of drinking water.

The present invention has been shown and described with reference to theforegoing exemplary embodiments. It is to be understood, however, thatother forms, details, and embodiments may be made without departing fromthe spirit and scope of the invention which is defined in the followingclaims.

1. A method, comprising: adding a metal salt reagent to water insufficient quantity and concentration to allow precipitation reactionsbetween the metal salt reagent and a dissolved contaminant in the waterto go to at least near completion and to leave unreacted metal saltreagent in the water; flowing the water through a serpentine pipeconfigured to produce more turbulent flow through bends in the pipe andless turbulent flow through straight-aways in the pipe; and then flowingthe water through a bed of moving filter media, wherein unreacted metalsalt reagent in the water reacts with the filter media to generate areactive metal oxide or hydroxide coating on the filter media to adsorbdissolved contaminants remaining in the water.
 2. The method of claim 1,wherein the flowing the water through the serpentine pipe comprisesmaintaining sufficient flow to inhibit deposition of solids,precipitates or particulates in the serpentine pipe.
 3. The method ofclaim 1, wherein the contaminant is phosphorus, arsenic, selenium oranother heavy metal and the metal salt reagent is ferric chloride,ferrous chloride, ferric sulfate or ferrous sulfate and the filter mediais sand.
 4. The method of claim 2, wherein the contaminant isphosphorous and unreacted ferric chloride, ferrous chloride, ferricsulfate or ferrous sulfate in the water entering the bed of moving sandprovides a molar ratio of iron to phosphorus of 5:1 to 40:1.
 5. Themethod of claim 2, wherein the contaminant is arsenic and unreactedferric chloride, ferrous chloride, ferric sulfate or ferrous sulfate inthe water entering the bed of moving sand provides a molar ratio of ironto arsenic of 100:1 to 200:1.
 6. The method of claim 2, wherein thecontaminant is arsenic and further comprising, before adding the metalsalt reagent, oxidizing the water to convert arsenite in the water toarsenate.
 7. A method comprising continuously regenerating a filtermedia by abrading the filter media sufficient to allow surface sites onthe filter media to be available for reacting with a chemical reagent,while simultaneously filtering contaminants from fluid flowing throughthe filter media and continuously adding the chemical reagent to thefluid supplied to the filter media for reaction with the surface sites.8. The method of claim 6, wherein the abrading scours the chemicalreagent and compounds containing the chemical reagent and thecontaminants from the filter media.
 9. A method comprising: introducingiron oxides into water supplied to a moving bed media filter effectiveto precipitate contaminants from the water and to form iron oxide coatedmedia surfaces in the moving bed media filter; simultaneously filteringthe precipitated contaminants with the moving bed media filter andsorbing other contaminants to the iron oxide coated media surfaces;abrading sorbed contaminant-iron solids from the iron oxide coated mediasurfaces; separating the precipitated and sorbed contaminants from theiron oxide coated media surfaces; and, continuously introducingadditional iron oxides to the moving bed media filter sufficient toregenerate the iron oxide coated media surfaces.
 10. The method of claim9, wherein the introducing iron oxides into water comprises introducingiron salts into the water in sufficient quantities to react withavailable water chemistries to form the iron oxides in quantitieseffective to precipitate a majority of the contaminants and to form theiron oxide coated media surfaces without significantly increasing ironconcentrations of effluent water obtained from the moving bed mediafilter.